When attempting to minimize the overall radar cross section (RCS) of an aircraft, RF radiators provide a special problem. They must, of course, be "exposed" in an RF sense else they cannot perform as desired to transmit and/or receive RF signals. However, such exposed radiators also can provide fairly efficient reflection of incident radar signals unless special precautions are taken. This problem is especially acute when large scale arrays of RF radiators are employed.
Industry has given considerable attention to low RCS (radar cross-section) antennas. The objective is to have the reflection coefficient at the input to a radiation aperture to be not only minimized but also uniform from element to element within the array. With previously available reciprocal phase shifters, this requirement has been very difficult because the reflection coefficient at each end of a phase shifter is separated by many wavelengths of reciprocal phase shift resulting in a reflection coefficient which varies significantly with phase state (i.e. the magnitude of phase shift programmed into a particular phase shifter).